Capital Stock - Issuances of Common Stock (Details) - USD ($)	3 Months Ended			12 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stock Issued During Period, Value, New Issues				$ 118,328,000	$ 83,275,000	$ 171,295,000
At the Market Offering Program [Member]
Share Price	$ 10.79	$ 10.48
Stock Issued During Period Shares New Issues	6,707,101	646,753	5,024,530	11,172,656	6,235,017
Stock Issued During Period, Value, New Issues				$ 118,328,000	$ 83,275,000
Value of Common Stock Available to be issued under At The Money Program				$ 15,000,000
At the Market Offering Program [Member] | 2015-Q1 [Member]
Share Price				$ 13.66
Stock Issued During Period Shares New Issues					1,210,487
Stock Issued During Period, Value, New Issues					$ 16,175,000
At the Market Offering Program [Member] | 2015-Q2 [Member]
Share Price				13.65
Stock Issued During Period Shares New Issues					5,024,530
Stock Issued During Period, Value, New Issues					$ 67,100,000
At the Market Offering Program [Member] | 2016-Q2
Share Price				$ 10.48
Stock Issued During Period Shares New Issues				646,753
Stock Issued During Period, Value, New Issues				$ 6,591,000
At the Market Offering Program [Member] | 2016-Q3
Share Price				$ 10.8
Stock Issued During Period Shares New Issues				3,818,802
Stock Issued During Period, Value, New Issues				$ 40,525,000
At the Market Offering Program [Member] | 2016-Q4
Share Price				$ 10.79
Stock Issued During Period Shares New Issues				6,707,101
Stock Issued During Period, Value, New Issues				$ 71,212,000